The present invention relates to chemical analysis using microwave assisted nuclear magnetic resonance (“NMR”). In particular the invention relates to determining the moisture and fat and oil content of samples using NMR and microwave radiation.
The present invention relates to chemical analysis of foodstuffs or other materials for which it is advantageous to know the moisture and fat (and oil) content of the material. For example, commercial production of processed foods such as hot dogs and cheese requires close control of fat and oil content. Variation in fat and oil content during the production process is detrimental to product quality and can adversely affect production economics. From a more positive standpoint, the fat content of a sample also provides useful information about food products such as texture, heat resistance, mouth feel, and flavor release. Additionally, many foods are subject to various statutory and regulatory labeling and content requirements with respect to the fats and oils they contain. Information about fat and oil content is often valuable or necessary in controlling various food processing techniques.
Those skilled in the art know that there is little chemical difference between fats and oils, the primary distinction being that fats are solid at room temperature and oils are liquid. Accordingly, the terms “fat” and “oil” may be used interchangeably herein.
Traditional methods for determining the moisture and fat content of foodstuffs include extensive drying and solvent based chemical extractions. These traditional methods are time consuming. The time lag inherent in the most widely used testing methods prevents production processes from operating at optimal efficiency. Furthermore, many methods require solvents that are expensive, often hazardous, and pose disposal problems. Accordingly, scientists have sought alternative means for determining fat and oil content in samples.
Since the late 1960's scientists have proposed using NMR as an alternative means of determining the fat and moisture content of foodstuffs. NMR analysis is essentially a spectroscopic method that measures a phenomenon that occurs when the nuclei of certain atoms are placed in a static magnetic field and then exposed to a second oscillating electromagnetic field. In very simplistic terms, during NMR analysis a substance is placed in a magnetic field that affects the “spin” of the atomic nuclei of certain isotopes of elements. The nuclei orient themselves in a specific way in response to the magnetic field. If a second radio frequency (RF) magnetic field (e.g., radio wave) is passed over the nuclei, the protons in the nuclei will be made to reorient or “flip” when the RF field reaches a specific frequency. When the RF field is turned off, the nuclei “flip” back releasing energy that provides data on the molecular structure of the substance. This back-and-forth orienting of nuclei is known as resonance.
NMR resonance occurs at different frequencies for different materials. For liquids, the frequency band for NMR resonance is relatively narrow. For solids, the frequency band is broader. This distinction in NMR resonance makes it possible to distinguish the protons of liquid from those of solid constituents of the sample. In this manner the percentage of liquid and solid in a sample may be determined.
In another type of NMR, referred to as “pulsed” NMR, a sample is exposed to a pulse of radio frequency (“RF”) energy that magnetizes (“flips”) the proton nuclear magnetic moments in the sample. Following the pulse, the protons return to their initial states (“relaxation”), but do so over a characteristic period of time (relaxation time) that reflects the chemical surroundings of the proton, and thus the composition of the sample. A representative (although not limiting) discussion of pulsed NMR is set forth in published U.K. Patent Application GB2261072A.
Furthermore, under proper circumstances, NMR can distinguish not only between liquids and solids, but also between chemical compounds. Theoretically, in abstract circumstances, all protons should resonate at the same frequency or relax over the same time period. Surrounding electrons, however, interfere with the magnetic field acting upon a given proton, and thus each proton will resonate at a slightly different frequency, or relax over a different time period, depending upon the electron density around it. As a result, different compounds (and different functional groups within compounds) have different resonance frequencies and different relaxation times. These differences are typically represented as graphical peaks along a spectrum that plots resonance frequency or relaxation time, depending upon the type of NMR being used.
As mentioned previously, NMR has long held promise as an alternative to solvent extraction for quantitatively determining the components of a sample. Efficiently utilizing NMR in this regard, however, has proven difficult. This difficulty is especially prevalent in determining the fat and oil content of foodstuff samples.
For example, NMR resonance occurs over a narrow band for liquids and this narrow window of NMR resonance is used to easily distinguish liquids from solids. Traditional fat and oil analysis takes advantage of this by melting all the fat and oil in a sample prior to NMR analysis. Because many foods have a relatively high moisture content, and because high moisture content usually makes NMR analysis unfeasible, food samples typically must be thoroughly dried prior to NMR analysis.
After the sample is dried, the sample is usually heated until all the fat and oil present in the sample is assumed to have melted, with the further assumption that the only liquid remaining in the sample is fat. Melting the fat and oil also means that a portion of the fat and oil may drain from the sample. As a result, when the sample is removed from the oven and prepared for NMR analysis, the sample no longer contains the same amount of fat and oil as did the original sample. As a further disadvantage, if aggressive heating techniques, such as very hot convection ovens or conventional microwave ovens, are used to speed drying of the sample, the chemical structure of the sample may be altered (e.g., meat is cooked) which may alter the NMR results and provide a less accurate— or even highly inaccurate—analysis.
One proposed method for avoiding the problems associated with conventional drying techniques is to chemically remove the water from the sample prior to conducting a NMR analysis. UK Patent Application GB 2,261,072 proposes to chemically remove the water from the sample through the addition of a drying agent such as calcium carbide or calcium oxide. The addition of these drying agents, however, must be based upon expected moisture content. Accordingly, errors in the addition of the drying agent are unavoidable. Such errors affect the weight of the sample and possibly the NMR analysis (if moisture remains in the sample) thereby providing inaccurate results for fat and oil content.
In short, current methods for determining fat and oil content possess a high degree of statistical uncertainty or are unsuitable for continuous production processes. Therefore, a need exists for a method and apparatus for determining the fat and oil content of samples that does not possess the inherent inaccuracy of known methods. Preferably this new method and apparatus will rapidly and accurately determine the fat and oil content of a sample thereby providing a more efficient overall production process.